Another benefit: Green roofs last longer than conventional ones. Where typical roofs might last 20 years, green roofs will survive 40 to 50 years. They protect the roofs waterproof layer from damaging ultraviolet rays and extreme day/night temperature fluctuations, which may cause cracks.
While the costs of green roofs are one-and-a-half to twice that of conventional roofs, advocates say green roofs make long-term economic sense, even without factoring in energy savings.
The rooftop green space also helps mitigate a phenomenon known as the urban heat island effect. The city microclimates occur when acres of densely-packed human-made building materials absorb heat energy from the sun.
"In Atlanta on a hot summer day, it might be 10 degrees [Fahrenheit/5.6 degrees Celsius] warmer in the city than out in the countryside," Rowe said. Green roofs can help mitigate the unnatural temperature rise, Rowe said.
In Germany, where green roof are most common, the city of Stuttgart has promoted green roof and facade planting through subsidies for nearly 20 years.
Martina Laun and Klaus-Juergen Evert serve in the city's park and cemetery department. They noted that the number of green roofs in the city is permanently increasing, thanks to a combination of subsidies and zoning laws, which require green roofs in certain areas.
Under a grant-aid program established by the Stuttgart City Council, private citizens are awarded grants to help cover construction costs.
Between 1986 and 2003, the plan subsidized some 50,018 square meters (540,000 square feet) of private green roofs at a total cost of U.S. $1,076,000. During the same period, the city also invested U.S. $2,523,000 for roof plantings on public buildings. The total area: 102,423 square meters (1.1 million square feet).
Laun and Evert noted that the program has fostered aesthetic benefits as well as environmental ones. "Nature's rhythm is made visible by seasonal changes in the vegetation, even in an urban roof environment," the pair wrote in an e-mail to National Geographic News. "In terms of design, roof planting represents a considerable aesthetic improvement and altogether a significant ecological benefit."
The green roof concept is winning more converts in the United States.
Ford Motor Company created a 10.4-acre (4.2-hectare) living roof, the world's largest, atop its Dearborn, Michigan, truck plant final-assembly building.
In Chicago, Mayor Daley is leading by example. City Hall sports a green roof that was the city's first. Over a hundred building projects, incorporating a million square feet (93,000 square meters) of green roof, are now underway. Officials are even experimenting with green roofs at O'Hare International Airport as a means to reduce noise.
At the Chicago Center for Green Technology, demos showcase green-roof technology to everyone from developers and builders to schoolchildren. The center also serves as a research center, where leading suppliers are invited to install different green-roof plots that are monitored and compared.
And the city government is providing at least a nudge to those involved in new building ventures. Projects that receive tax-increment financing support from the city are required to include some level of green roof.
"I hate to say that we're mandating, but we're really talking to big box [stores], architects, contractors, developers, and others about how important [green roofs] are to the environment and to business," Daley said.
The mayor believes that the city's cleaner, greener roofs are an important part of long-term environmental planning close to home.
"The environmental movement often seems like it's happening somewhere else and people forget about our own community," he said. "We need to be sure that we're planning well."
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